The last thing I would want for a potential employer to know is that I was in a self-confidence crisis or that I might even be right that I wasn't a good developer.
Wouldn’t be the first time
One explanation is that she wants to convince you that interviews are broken and that you need to buy her book (or hire her as a consultant to improve your hiring process).
Another explanation is that thanks to her decades of experience in the field, she honestly believes that she can help you nail these interviews if you know what to learn, so she created the books to help you prepare for the interviews, and get well paid and (potentially) interesting jobs.
In the first, she makes up a problem only to sell you the solution. In the second, she identified the problem, and created products to help you overcome this problem. When both are reasonable explanations, I like to give people the benefit of the doubt.
I don't see how it's "less than ideal" to end a contract job when the contract's up; even if it's contract-to-hire.
And I don't see how a 3-month lacuna in your resume makes it "less than ideal". I took four years out when my kids were small. Oh, I worked - I learned new languages, burnished my skills, and made a little money selling some software I had written; but on my resume, it looked like I'd taken a 4-year holiday, which I'd dressed-up badly as some kind of freelance work.
FWIW, I'm not a good programmer either, despite having spent most of my career programming. I'm fine with a block of code, or a module; but beyond a certain level of complexity, I don't seem to be able to produce clear designs.
Didn't stop me getting jobs.
Note that the Substack owner (Gayle) has an incentive to convince you that tech hiring is broken; She is the author of a famous book about how to master tech interviews. This anecdote about someone quitting a contract job 3 months in, putting a 4-5 month gap on their resume, and then wondering why they’re not immediately getting job offers from the first companies she applies to in the middle of a huge tech recession isn’t really a mystery. It’s presented as if the person is merely being unfairly rejected by bad interview processes. The implicit story is that you’re supposed to buy the Substack owner’s book about coding interviews to overcome this problem, but the anecdote author is going to have to tap her network and apply to a huge number of jobs to overcome her less than ideal resume.